The present invention relates to temperature sensors and, more particularly, to temperature sensors adapted for use in an environment containing microwave radiation.
U.S. patent application Ser. No. 779,285, now U.S. Pat. No. 4,681,122 the teaching of which is incorporated herein by reference, discloses apparatus for microwave thermotherapy useful in the treatment of cancerous tissue, particularly in the brain. The disclosed apparatus includes a catheter having at least two lumens extending along a substantial length thereof. The catheter is inserted into the tissue to be treated to position a microwave antenna in one of the lumens within, or very close to, the cancerous tissue. The microwave antenna is fed microwave energy of sufficient intensity and for sufficient time to damage the cancerous tissue without damaging surrounding healthy tissue.
Effective treatment requires that the cancerous tissue in the vicinity of the catheter be held at a temperature within about half a degree of 43 degrees C. for a period lasting on the order of an hour. A temperature substantially below this range is tolerated by the cancerous tissue without significant damage, and thus the treatment fails. A temperature substantially above this range can cause unnecessary damage to healthy tissue without significant improvement in the treatment result. Such fine temperature control is complicated by the fact that the amount of microwave radiation required to maintain the desired temperature decreases over a treatment period due to a reduction in the ability of heat-damaged cancerous tissue to maintain fluid circulation.
As a consequence of the preceding, the above-referenced patent application employs a thermistor in the second lumen of the catheter positioned to measure the temperature of the tissue being heated by the microwave energy. This requires that the thermistor and its leads be within the influence of microwave energy radiated by the microwave antenna. It has been discovered that the microwave energy tends to heat the thermistor chip and its leads, whereby higher temperatures are measured than would actually be measured if the only source of heat was the tissue being treated. Unless dealt with in some manner, such false temperature readings may lead to the maintenance of tissue temperatures outside the range permissible for effective treatment.
One method of dealing with microwave heating of the thermistor and its leads includes estimating the component of temperature measurement produced by the microwave energy impinging on the thermistor and its leads. This is possible in the apparatus of the referenced patent application since the geometry in the catheter is fixed and the amplitude of the microwave signal is known. Based on empirical measurements, the measured temperatures may be calibrated to yield close approximations of the actual tissue temperatures. A further method includes periodically turning off the microwave radiation to permit the temperature of the thermistor chip and its leads to reach an equilibrium temperature due solely to the tissue temperature. This technique has the drawback that tissue cooling inevitably takes place during the off period, and the rate of cooling may not be known precisely. As a consequence, the temperature measured near the end of the off period is lower than the tissue temperature during microwave radiation.
A need thus exists for a technique permitting temperature measurement simultaneously with microwave heating of tissue without the measured temperature containing substantial errors due to microwave heating of the temperature sensor.